1. Field of the Invention
This invention relates to a device for driving a moving body to a given position with a motor and more particularly to a device suited for driving a lens to a given position in adjusting the focus of a camera.
2. Description of the Related Art
A DC motor is often employed for lens driving in an automatic focusing device which is provided within a camera for automatic focal length adjustment. For this purpose, the motor is arranged to give a high degree of torque by means of a reduction gear despite of the compact size thereof. Meanwhile, a vibratory-wave motor which is capable of giving a high torque without recourse to a reduction gear and has a high degree of responsivity has recently appeared to be used for driving a focusing lens in place of the DC motor.
A servo device such as the above stated lens driving device for focus adjustment is generally arranged to perform closed loop control on the basis of information fed back by using an electronic control device of the kind using a computer (hereinafter may be called a microcomputer). Therefore, the lens driving device is arranged to detect a change in the rotating speed of a motor-driven lens system, etc.; to have a detected degree of control fed back to be reflected upon a change in a voltage applied to the motor. Meanwhile, the device using the vibratory-wave motor is generally arranged to have the detected degree of control fed back to be reflected upon a change in the frequency of a power supply to the motor.
In case where the driving control over the lens system of a camera is to be changed by an electronic control device using a microcomputer, the degree of the change is determined in the designing state according to the kind, required performance, etc. of the camera. Generally, however, the device is designed to permit selection of rotational frequency (rotation speed) from among 16 to 128 steps of values.
The lens driving device such as the above stated automatic focusing device or the like is to be used for forming the image of a photographing object on a film surface (or on a CCD or the like) in a focused state during a photo taking operation. Therefore, the device is not only required to permit accurate focusing for a sharp picture but also is preferably arranged to permit the photo taking operation to be carried out without being affected by any vibrations.
In the case of the above stated lens system driving device using a DC motor, the actual rotational frequency of the motor does not change immediately upon the instructions of the microcomputer for changes in the rotational frequency so that the driving control can be smoothly accomplished. On the other hand, however, the performance of the driving device of this kind is inferior in accurately bringing the lens system to a stop in a given position.
Meanwhile, the lens driving device of the kind using the above stated vibratory-wave motor is advantageous in that the device has a relatively easy operability for rotation speed control and yet has a high degree of responsivity to speed control. While the device has the advantage of, for example, accurately stopping the lens system in a given position by virtue of its high degree of responsivity to the instruction of the microcomputer for changing the rotational frequency of the motor, a disadvantage of the device lies in that: The high degree of responsivity tends to bring about vibrations as a result of its rapid response to the instructions of the microcomputer to change the rotational frequency. The lens driving device of this kind, therefore, presents a problem in driving control for a smooth change in the rotation speed.
The problem of the lens system driving device using the vibratory-wave motor becomes more pronsonced in case where a power-focusing lens system driving device is arranged in combination with the above stated automatic focusing device. Since this device has the high degree of responsivity and is advantageous as a driving device of a camera for focus adjustment or the like, a solution of this problem is highly desirable.